In many industrial systems, large vessels are used for various purposes. In many instances, these vessels contain materials which are at high temperature, high pressure or both and are vulnerable to failure. As a result, it is necessary that the inside of such vessels be inspected periodically. In the past, such inspections have been accomplished by simply emptying the vessel and physically inspecting the interior surfaces of the vessel. This permitted the detection of bulges, cracks, and pits and allowed for visual observation of the inside of the vessel. Some such vessels may be as large as twenty to thirty feet or more in diameter and fifty to seventy feet or more in height. One example of such vessels is petroleum coking vessels. These vessels are exposed to both thermal and mechanical stresses and are typically periodically inspected. The invention will be discussed primarily by reference to delayed petroleum coking vessels although the invention is equally useful with other large vessels.
Such vessels can be inspected by emptying the vessel and erecting scaffolding in the interior of the drum so that inspection personnel can make essentially manual inspections, including dimensional measurements to detect changes in the shape of the drum structure, visible defects and the like. Not only is this expensive and time consuming, inspection personnel are exposed to risks of released gases, falls and the like during the inspection process. Subsequently methods were developed, for instance as disclosed in U.S. Pat. No. 5,425,279 issued to R. D. Clark, et al on Jun. 20, 1995 (the '279 patent), which is hereby incorporated in its entirety by reference, for inspecting the interior of vessels using laser devices with reflective laser light being measured to detect and define bulges and other dimensional changes inside the vessel. This laser inspection technique was combined with the use of a video camera with both the laser system and the video camera being operable to provide recorded data by reference to the specific portions of the vessel tested. This data permitted the detection of bulges and the like and visual examination of the inside of the vessel. Unfortunately, since the vessels are not readily cleaned as they can be when inspection personnel enter the vessel, there may be coatings of material over the surfaces which prevent the detection of corrosion pits and the like in the interior surface of the vessel unless the pits or other irregularities have reached a size such that they can be detected by the laser system.
Coke drums typically have an outer shell of carbon steel or alloy steel ¾ to 1½ inches wall thickness and are internally clad with a layer of ferromagnetic stainless steel (410 stainless steel, for example) which can be up to 2.5 mm thick. The drums contain girth welds with weld caps up to two inches wide. The caps are normally low profile.
Before any inspection, the coke is cut from the internal surface of the drum using a high-pressure water cutter. This usually provides a relatively clean surface to enable the visual identification of crack indications.
Although very shallow craze cracking can exist in the liner, which does not in itself cause a problem, some cracks can grow to a significant depth and even penetrate into the substrate shell. While visual indications of surface cracking can be identified using the visual system, it is not currently possible to classify the cracking for depth in order to assess the severity of the cracking. Currently insulation has to be removed from the outside of the drum and ultrasonic testing is used to try and assess the crack depth. Apart from the cost involved in removing and replacing insulation, the ultrasonic testing is hampered by the cladding interface. Since these cracks can result in failures if severe, it would be highly desirable to determine the severity of the cracks, especially around weld areas, more easily and reliably and even more desirably in conjunction with the visual and laser testing.